1. Field of the Invention
The present invention relates, in general, to a method for forming a field oxide of a semiconductor device and, more particularly, to a method by which the ungrowth of field oxide is prevented and a gate oxide enhanced in reliability can be produced.
2. Description of the Prior Art
In order to better understand the background of the invention, a description will be given of the conventional method of forming a field oxide in conjunction with figures.
Referring to FIG. 1, there are illustrated processes of forming a field oxide, according to the conventional technique.
First, as shown in FIG. 1a, prepared a semiconductor substrate 1 is over which a pad oxide 2 and a first nitride 3 are sequentially formed, through which the first nitride 3 with a mask at a predetermined field region is etched. At this time, the first nitride 3 is over-etched in such a way that the semiconductor substrate 1 is recessed to a predetermined thickness, for example, about 50-100 Angstrom.
On the resulting structure a second nitride 4 is then deposited, as shown in FIG. 1b.
Subsequently, the whole surface of the second nitride 4 is subjected to dry etching without any mask, to form a nitride spacer 4', as shown in FIG. 1c.
A further dry etching process is executed to recess the exposed region of the semiconductor substrate 1, as shown in FIG. 1d. At this time, since the etching selection ratio of silicon to nitride has a certain value, the nitride 3 of the active region, besides the semiconductor substrate 1 of the field region, is partially etched to create nitride residues R, which are collected in the recess. At a site with a relatively small area ratio of active region to field region, such as the cell region of a memory device, the nitride residues are produced at a small amount. But, a significant amount of the nitride residues is produced in a site which has a much larger area of active region than that of field region, for example, at a peripheral circuit region. The nitride residues R produced in the silicon recess etch process are either released or redeposited on the gorge shape field region. As an example of the latter case, because such residues occur at a significant amount at a field region of a peripheral circuit site, part of the residues remain on the bottom of the field region.
FIG. 1e is a cross section after an additional dry etch is performed to remove the residues. However, as seen, the nitride residues are not completely removed by the additional etch but remain thin and partial.
When field oxidation is applied under this state, the following properties appear depending on oxidation conditions. Herein, because it takes too much time for dry oxidation, the properties are those which are obtained on the assumption that the field oxidation is executed in a manners only.
When a part of the nitride residues R remain, a wet field oxidation using hydrogen and oxygen allows the normal growth of a field oxide 5 as shown in FIG. 2a. This is because wet oxidation has an excellent ability to oxidize nitride components.
When a dry field oxidation using oxygen alone is carried out, the field oxide thus obtained has an abnormal shape, owing to the residues, in that the field oxide does not grow at the central portion, as shown in FIG. 2b. This is attributed to the fact that dry oxidation is far inferior to wet oxidation in oxidizing the nitride residues R.
However, at a site with a large ratio of active region to field region, even dry field oxidation can make the field oxide grow normally because there are no nitride residues, as shown in FIG. 2c.
FIG. 3 illustrates different points in the shape of the field oxides which are formed in wet and dry field oxidation manner.
FIGS. 3a and 3b show the field oxides which are obtained by carrying out a wet oxidation process at a temperature of 950.degree. C. and 1,100.degree. C., respectively, while the field oxide of FIG. 3c is obtained by carrying out a field oxidation at a temperature of 1,100.degree. C. in a dry manner. As seen, while the field oxide under a nitride spacer has a negative slope at such a low temperature of 950.degree. C. Even at a relatively high temperature of 1,100.degree. C., the slope of the field oxide obtained becomes near zero. In the case that the slope of the field oxide is negative or zero, when a gate oxide is formed after the removal of the nitrides 3 and 4 and a sacrificial oxidation process, an electric field is focused on the boundary between the field oxide and the gate oxide, leading to a degradation of the reliability of the gate oxide.
In FIG. 4, the reliability of the gate oxide obtained under conventional wet field oxidation is shown. As shown in FIG. 4, the reliability is poor because the slope of the field oxide is negative or zero.
Dry field oxidation at 1,100.degree. C. gives a positive slope to the field oxide 5 under a nitride spacer, as shown in FIG. 3c. In this case, the phenomenon where an electric field is focused on the boundary at which the field oxide meets the gate oxide is prevented.
As afore illustrated, carrying out dry field oxidation in the presence of the nitride residues, by-products which are produced in a significant amount in a large active region-to-field region site such as a peripheral circuit region when forming a field oxide, results in abnormally growing the field oxide at the center of the field region. Further, when the conventional wet field oxidation process is executed, the field oxide has a negative or zero slope at the boundary to the active region, degrading the properties of the gate oxide.